A generally known method for manufacturing a thermoplastic sandwich structure comprises extruding a thermoplastic foam layer and adhering it as a foam core between two fiber-reinforced thermoplastic cover layers, usually from the same thermoplastic as the foam layer. A drawback from this known method is that foaming of the core and adhesion thereof to the cover layers are separate steps, requiring strict process control and alignment.
EP-A-636463 has disclosed a batch-wise method of producing a sandwich plate that consists of a foamed core layer and two fiber-reinforced cover layers. This known batch process comprises the steps of providing a sheet of a thermoplastic material, such as polyetherimide, containing an amount of a suitable solvent; placing said sheet between two fiber-reinforced layers of a similar thermoplastic material, such as polyetherimide; placing the core web and cover layer assembly between two pressure plates; supplying heat and pressure to the pressure plates to cause foaming of the core web and cooling the pressure plates when a predetermined foam layer thickness is obtained. In the examples foaming is performed at a temperature below Tg of polyetherimide containing dichloromethane as a solvent functioning as a physical blowing agent. A drawback of this known batch wise process is the relatively slow production rate in particular for producing sandwich structures. Another drawback is that the solvent needs to be removed from the sandwich structure by drying, which is a time and energy consuming step adding to the costs.
GB 701 066 has disclosed a batchwise method of producing closed cell cellular bodies from thermoplastic masses, such as polyvinyl chloride by dissolving gases under high pressure in the mass in a closed press, followed by heating until the mass gelatinizes completely, then reducing the pressure and increasing the volume of the mass by ⅕-⅖ of the initial volume and thereafter cooling the mass, relieving the mass completely of pressure and freely expanding the mass. This method is not suitable for production of sandwich structures, because upon freely expanding the cover layers would tend to bulge and as a result these cover layers would not present a flat and smooth surface.
U.S. Pat. No. 3,160,689 has disclosed a method of batchwise producing a closed cell cellular body starting from a plastigel e.g. from polyvinyl chloride comprising softeners and/or solvents, and a latent blowing agent, kept in a putty-like consistency, preferably by the addition of stiffening gelling agents. This plastigel having the shape of the body to be made on a reduced scale, is placed in a pressure vessel under superatmospheric gaseous pressure and subjected to heating in order to decompose the latent blowing agent and gelatinize the plastigel. The gelatinized body is then cooled to a temperature below the gelatinization temperature while maintaining its plastic condition, whereafter pressure is relieved thereby expanding the body to a final shape without contacting the mould walls. In a preferred embodiment a limited expansion of the gas distributed in the shaped body is performed prior to cooling. Again this method is not suitable for production of sandwich structures, because the cover layers would bulge and not present a flat and smooth surface. Moreover an expansion in all directions of the body to be foamed cannot be applied to the reinforced cover layers.
Furthermore, the addition of the gelatinizing material reduces the mechanical properties of the foam and thus the compression/shear strength of the final foam. In a sandwich structure however, the mechanical performance of the foam is very important for the final mechanical performance of the sandwich structure. Thus gelatinizing material should be avoided. Also the presence of a substantial amount of solvents is detrimental for its application in a sandwich structure, because solvents have to be removed by a drying, e.g. in an oven, which is a time-consuming process as remaining solvent in the foam has to diffuse through the cover layers. Additionally, the incorporation of solids in the foam layer is undesirable for light-weight products.